Problem: Solve for $a$ : $a + 9 = 18$
Explanation: Subtract $9$ from both sides: $ \begin{eqnarray} \\ a + 9 &=& 18 \\ \\ {-9} && {-9} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{9 + a} &\hphantom{=}& \hphantom{18} \\ a &=& 18 {- 9} \\ \end{eqnarray} $ Simplify. $ a = 9$